The manufacture of fabrics and textiles is a complex process. Generally speaking, fabrics are produced from yarns. A yarn is a continuous, often plied, strand composed of either natural or man-made fibers or filaments. In constructing a fabric, yarns are fed to a loom where they are weaved together. In the loom, the longitudinal yarns are called warp while the transverse yarns are called filling. Modern looms operate at high speeds, stretching the warp. In order to strengthen the warp, to reduce friction between the warp and the loom, and to increase weavability, the warp is pretreated with a sizing material. The sizing material is applied to the yarn by a slasher. Sizing materials are adhesive liquids that penetrate yarn pores forming a film on the outer diameter of the yarn. The size fills the pores and thus reduces absorption of subsequently applied adhesives or coatings. The size creates smooth surfaces by laying down projecting fibers and creates sufficient strength to resist the strains of weaving by fastening the fibers together. After applying the size, however, the yarn must remain flexible enough to be woven. An ideal size will eliminate loom warp stops and, at the same time, add to the overall quality of the fabric.
An effective size formula must impart several basic characteristics to a yarn in order to reduce warp breaks during weaving.
Perhaps the most important property of a warp yarn is the longitudinal breaking strength or tensile strength. Also important is the tearing strength or, in other words, the strength of the yarn in the perpendicular direction. The yarn must be strong enough to withstand the tension exerted on it by the loom. A size increases the strength of a yarn by cementing fibers together and by adding a film to the outside diameter of the yarn.
Although yarn strength is important, it is also critical that the yarn remain flexible. Consequently, the size must create an adhesive coating on the yarn that is pliable. Flexibility is important during weaving operations when the yarn is threaded over and around other yarn. During weaving, the yarn is repeatedly bent in many directions. If the yarn were not flexible, it may snap or break during operation of the loom. If the film formed by the size were not flexible, it may rupture and shed during weaving, thus creating weak spots in the yarn, increasing friction between the yarn and the loom, and create dusting which could decrease the efficiency of the machinery.
Two indicators of flexibility are elongation and elasticity. Elongation is a measure of the amount of stretch a yarn will undergo before it breaks. Elasticity gauges the ability of the yarn to return to its original length after being stretched. Flexibility, elongation and elasticity can be increased by maintaining moisture in the sized yarn. Moisture retention can be controlled by adding a humectant to the size solution, by not overdrying the yarn after it has been sized and by weaving under humid conditions. However, the sizing formula still prevails in determining the overall flexibility of the sized yarn.
Another important property of a sized yarn is its ability to weave without creating high friction between yarns and between the yarn and the machinery. The coefficient of friction of warp yarn is highly dependent on the properties of the size film. High friction will inevitably lead to shedding and yarn breaks. Friction can be reduced if the size can lay down projecting fibers of the yarn during formation of the film. If projecting fibers are present, they will shed off on the loom, creating fuzz balls or lint buttons.
In order to create a yarn with the basic characteristics as discussed above, the size formula must exhibit certain properties. For instance, the size must be adhesive to yarn. Adhesion is not only necessary for cementing fibers together, but is also important for the film to adhere to the yarn. The sizing agent must also adhere uniformly to the yarn. Film consistency ensures uniform strength over the length of the yarn.
Another property of the size to consider is its ability to penetrate the yarn during application. Penetration must be strictly controlled during processing. If the size does not penetrate the yarn sufficiently, the film will not adhere adequately. On the other hand, if the size penetrates and soaks the entire yarn diameter, the resulting warp will be too stiff.
Directly related to penetration is the viscosity of the size formula. Low viscosities lead to excessive penetration which results in stiffness and poor film forming ability. The viscosity or thickness of the size formula should be such that a proper film forms that is capable of laying down the projecting fibers. Thickening agents can be added to the sizing agent in order to increase viscosity.
Another important consideration is the removability of the size. Once yarn has been sized and woven, the fabric must be desized. A size, therefore, must be easily removable from the yarn. Desizing solutions are often expensive, and may harm the fabric.
Spent desize solution is usually discharged to a wastewater stream which is treated and then released to the environment. The wastewater stream is subject to environmental regulation. Recently, the federal government has set much more stringent standards and discharge limits. A size formula which can have an adverse impact on the environment may create a large cost to the discharger for treatment and disposal. For example, in 1980, a large textile mill reported wastewater treatment operating expenses of over $200,000.
Typical wastewater pollutants resulting from sizing agents include biological oxygen demand (BOD), chemical oxygen demand (COD), and aquatic toxicity. BOD is measured by an empirical test in which standardized laboratory procedures are used to determine the relative oxygen requirements of wastewater. The test measures the oxygen required for the biochemical degradation of organic material and the oxygen used to oxidize inorganic material such as sulfides and ferrous iron. COD, which is related to BOD, is a measure of the oxygen equivalent of the organic matter content of a sample that is susceptible to oxidation by a strong chemical oxidant. For BOD and COD, the higher the test result, the greater the pollutant.
Pollution has become a primary concern when formulating a size. Recently, a substantial amount of research and development has focused on finding a size formula that is less environmentally hazardous. Choosing an environmentally safe size would not only result in substantial savings to the manufacturer but would also be very beneficial to the environment.
A size formula can contain many ingredients and additives. The formulation of the size depends mostly on the fibers being treated. However, each size formula contains a base material, or a sizing agent, that is selected principally for its film forming ability. If other additives are added to the size, the sizing agent must be compatible with them. Incompatibility could result in the presence of precipitates or in-phase separation of the size solution thereby causing poor results.
The most popular known sizing agents include starches, carboxymethyl cellulose, polyvinyl alcohol (PVA) and mixtures thereof. Starch is a carbohydrate, synthesized within plants by combination or polymerization of dextrose. Physical properties such as pH, viscosity, moisture content, adhesiveness and film forming ability may vary from each source and supply. Starches also can be chemically modified. Specifically, starches can be acid modified, ethoxylated or oxidized. Starch, in granular form, is insoluble in cold water but produces a uniform viscous fluid when heated beyond a certain critical temperature. The viscosity of a starch and water mixture varies with temperature, and the difference in viscosity with rise in temperature is characteristic of each starch. The term "starch" is used herein as a generic term to describe all types of starch in general, unless otherwise denoted. Starches are relatively inexpensive. However, starches have a limited shelf life and usually require the above-described chemical modification in order to be compatible with other sizing agents and additives. Starches also tend to add a lot of weight to the treated yarn. The largest drawback to using starch, however, is its high BOD and COD.
Carboxymethyl cellulose is made from cellulose derived from wood pulp that is impregnated with sodium hydroxide to form alkali cellulose. This product is then combined with sodium monochloroacetate to form sodium carboxymethyl cellulose. Several types and grades of carboxymethyl cellulose can be produced. Carboxymethyl cellulose can create a tough film that retains water and is, therefore, flexible. However, carboxymethyl cellulose is more expensive than starch and frequently causes pollution problems in the waste stream.
PVA is a synthetic polymer resin. PVA can produce a stronger yarn with greater elongation and lower stiffness. However, because it is a petroleum-based product, PVA tends to be relatively more expensive than other sizing agents. Although lower than starch or carboxymethyl cellulose, the BOD and COD values for PVA can be high. PVA is often used in combination with starch.
Excessive BOD and COD effluents from mills must be reduced to protect the environment. Among the most commonly used textile size materials are starch, PVA and carboxymethyl cellulose which have high BOD and COD values. Some common size materials have the BOD's listed below:
TABLE IV ______________________________________ FIVE-DAY BODs OF SIZE MATERIALS SIZE MATERIAL BOD (ppm) ______________________________________ Ahco nylon warp size 340,000 B-2 gum (starch dextrins) 610,000 Brytex gum 745 (starch) 610,000 Carboxymethyl cellulose (CMC) 30,000 Elvacet (polyvinyl acetate, PVAc) 10,000 Elvanol (PVA) 10,000 to 16,000 Globe Easyflow starch 650,000 Hydroxyethyl cellulose (HEC) 30,000 KD gum (starch) 570,000 Keofilm No. 40 (starch) 550,000 Morningstar starch 470,000 Nicol starch 570,000 Pearl (cornstarch, No. 173 and PT) 500,000 Penrod Gum 300 (starch ether) 360,000 RTC gum (starch-urea) 120,000 Starch No. 450 460,000 Sodium alginate 360,000 Wheat starch 550,000 Ambertex M (starch paste) 20,000 ______________________________________
Although various agents have been employed to size yarn, the particular features of the present invention have not heretofore been known. The prior art is generally deficient in affording an economical and effective sizing agent that is environmentally safe. The present invention overcomes the shortcomings of the prior art in that the composition and process disclosed herein result in an effective sizing agent for yarns that is environmentally safe for later desizing and discharging operations.